The purpose of this domain is to reserve a segment of the DNS
name space reserved for the use of museums; a name space whose
conventions are defined by the museum community. The
museum TLD grants users a quick and intuitive way to
verify the authenticity of a museum site. Conversely, since it is a
type of formal third-party certification, museums using this name
space obtain a way to assure visitors of the site's validity.

In addition to the eligibility requirements specified in the
museum charter, a number of naming conventions apply to
the labeling of subdomains. Extensive support is also being
introduced for internationalized domain names, as
described at about.museum.

Definition from Wiktionary, a free dictionary

Translingual

Pronunciation

(Internet) An
Internet TLD reserved for the
exclusive use of museums,
museum associations, and individual members of the museum
profession, as these groups are defined by the International Council of
Museums (ICOM).